U.S. Pat. Nos. 4,582,396 to Bos et al., 4,541,691 to Buzak, 4,583,825 to Buzak, 4,635,051 to Bos ("'051 Patent"), 4,674,841 to Buzak, and 4,719,507 to Bos ("'507 Patent") disclose a zero to substantially half-wave variable optical retarder that is a liquid crystal cell of a type described therein; they also disclose optical display systems using one or more of such retarders.
FIG. 1 is a plan view, and FIG. 2 is an elevational side view, of a first prior art liquid crystal cell 10 of that type, two of which are used as the two variable optical retarders in a system such as that described in the '051 Patent to produce a perceived image in a full range of colors from a raster display generated by a cathode-ray tube having a phosphor that emits light of many wavelengths including those of three primary colors. With reference to FIGS. 1 and 2, cell 10 has a transparent frontplane substrate 12 and a transparent backplane substrate 14 spaced apart from and generally parallel to each other. A thin layer 16 of nematic liquid crystal material is captured between substrates 12 and 14.
Frontplane substrate 12 has formed on its inner surface 18 plural split, nonintersecting, adjacent frontplane electrodes 20A-20E formed of an electrically conductive and optically transmissive material such as indium tin oxide. Adjacent pairs of frontplane electrodes 20A-20E are separated along their lengths and in part defined by split lines 22. Each of frontplane electrodes 20A-20E has a connection end 24A-24E and a distal end 26A-26E. Each frontplane electrode connection end 24A-24E has an electrode connection area 28A-28E; a different one of connectors 30A-30E is attached to a respective one of connection areas 28A-28E to provide independent electrical connection from each one of frontplane electrodes 20A-20E to a corresponding one of plural drive signal lines 32A-32E, which lead to a switching control unit, and receive signals, described in connection with FIGS. 3 and 4.
Backplane substrate 14 has formed on its inner surface 34 plural split, nonintersecting, adjacent backplane electrodes 36A-36E also formed of an electrically conductive and optically transmissive material such as indium tin oxide. Adjacent pairs of backplane electrodes 36A-36E are separated along their lengths and in part defined by split lines 37. Each of backplane electrodes 36A-36E has one of connection ends 38A-38E and distal ends 40A-40E. Each backplane electrode connection end 38A-38E has one of electrode connection areas 42A-42E; a different one of connectors 44A-44E is attached to a respective one of connection areas 42A-42E to provide independent electrical connection from each of backplane electrodes 36A-36E to a corresponding one of plural common signal lines 46A-46E, which are tied together and lead over line 48 to the switching control unit, and receive a signal, described in connection with FIGS. 3 and 4.
Frontplane electrode connection ends 24A-24E and backplane electrode distal ends 40A-40E are offset relative to each other, as are frontplane electrode distal ends 26A-26E and backplane electrode connection ends 38A-38E, to provide space for convenient attachment of connectors 30A-30E and 44A-44E. Each of frontplane electrodes 20A-20E overlaps with a corresponding one of backplane electrodes 36A-36E to define a respective one of plural cell segments 50A-50E. Each of cell segments 50A-50E is independently electrically driven over line 48 and a respective one of lines 32A-32E to impart to light propagating through the volume of liquid crystal material 16 in that cell segment a desired degree of optical retardation as described in the '051 Patent.
A region of overlap between frontplane electrodes 20A-20E and backplane electrodes 36A-36E defines a display area 54. Frontplane and backplane substrates 12 and 14 are assembled and held in place by suitable structures (not shown) so that each of frontplane electrodes 20A-20E opposes a corresponding one of backplane electrodes 36A-36E across liquid crystal layer 16 and so that each frontplane split line 22 is aligned exactly with a corresponding backplane split line 37 when cell 10 is viewed from and substantially perpendicular to its front. Split lines 22 and 37 are made as narrow as possible to avoid creating gaps in the display formed by cell 10. Frontplane and backplane electrode connection ends 24A-24E and 38A-38E are opposed across respective cell segments 50A-50E and across display area 54.
FIG. 3 shows a switching control unit 56 to which lines 32A-32E and 48 are connected. Control unit 56 produces in the proper sequence on respective ones of lines 32A-32E and 48 drive signals V.sub.DRIVE (A-E) (produced by a first driver 56') and common signal V.sub.COMMON (produced by a second driver 56"), each having a respective electrical potential that varies with time. Control unit 56 is connected to other components (not shown) of an optical display system such as described in the patents identified above and produces signals V.sub.DRIVE (A-E) and V.sub.COMMON in proper synchronization with those other components to produce an optical display system of the desired type.
FIG. 4 shows a signal V.sub.DRIVE that is applied to frontplane electrodes 20A-20E and a signal V.sub.COMMON that is continuously applied to all of backplane electrodes 36A-36E during display operation. Whenever V.sub.DRIVE and V.sub.COMMON are phase-displaced by 180.degree., the rms potential difference across a cell segment is nonzero and thereby places it in a field-aligned or "ON" state to impart essentially no optical retardation to light passing therethrough. Whenever V.sub.DRIVE and V.sub.COMMON are in phase, the potential difference across a cell segment is zero and thereby places it in a partly relaxed or "OFF" state to impart an amount of optical retardation to light passing therethrough. Switching control unit 56 (FIG. 3) sets the phase-displacement .phi. between V.sub.DRIVE and V.sub.COMMON at 180.degree. and then at 0.degree. in sequence to successive ones of frontplane electrodes 20A-20E at times appropriate to produce the desired full color display in accordance with the '051 Patent.
Cell 10 of FIGS. 1 and 2 has practical disadvantages. First, alignment of split lines 22 and 37 is critical when cell 10 is assembled; if they are not aligned, the viewing quality of an image seen through display area 54 is degraded. Second, forming split lines 22 and 37 takes time, and errors in that step decrease manufacturing yield.
However, it has been the prevailing view that in cell 10 split lines 37 should separate adjacent pairs of backplane electrodes 36A-36E, even though all the backplane electrodes are driven with the same signal V.sub.COMMON. Such split lines prevent electrical current from crossing among backplane electrodes 36A-36E from a cell segment that is an "ON" segment to other cell segments, particularly to cell segments that are "OFF" segments or are making a transition from the "ON" state to the "OFF" state. Such crossing electrical currents create unwanted differences in electrical potential across liquid crystal layer 16 and thereby impair the formation of the desired image, at least as to some of the pixels of that image derived from pixels of the underlying rastered display.
If backplane substrate 14 were provided not with backplane electrodes 36 separated with split lines 37 but instead with a continuous, conductive region (not shown) on surface 34, voltages induced in that continuous conductive region resulting from currents in it, caused by the electrical potential V.sub.DRIVE (A-E) applied to frontplane electrodes 20A-20E, would cause sufficiently large changes in electrical potentials across liquid crystal layer 16 to degrade performance of cell 10 to an unacceptable level. As an example, when a continuous backplane electrode is substituted for backplane electrodes 36A-36E in cell 10 (FIGS. 1 and 2), and that modified cell 10 is driven with differential drive waveforms, pixels in the corners of the displayed image may not display the proper images.
A second prior art liquid crystal cell (not shown) of the type described in the patents identified above was used in a system such as that described in the '507 patent to produce a perceived image producing a stereoscopic effect from a raster display generated by a cathode-ray tube. The second prior art cell was similar to cell 10 but had only two frontplane electrodes separated by one frontplane split line and only two backplane electrodes separated by one backplane split line. The left end and right end of each frontplane electrode had electrical contacts connecting those ends to a drive signal such as V.sub.DRIVE (FIG. 4) for that frontplane electrode. The backplane electrodes each had a single electrical contact located in respective top center and bottom center locations, and those two electrical contacts were electrically tied together and connected to a signal such as V.sub.COMMON (FIG. 4). This second prior art cell has the yield disadvantages discussed in connection with cell 10.
U.S. Pat. No. 4,652,087 to Bos et al. ("'087 Patent") discloses a method and apparatus for reducing optical cross talk in a liquid crystal optical switch. The system disclosed in the '087 Patent has a frontplane substrate supporting two electrodes divided by a split line and a backplane substrate supporting a backplane electrode that is common to all the electrodes on the frontplane substrate, i.e., a backplane electrode having a continuous, conductive region without a split line. However, that system requires more complex and expensive electronic circuitry than that needed to produce the waveforms of FIG. 4 that were used with cell 10.
There is thus a need for a liquid crystal cell of the type described in the patents identified above that can be manufactured more efficiently than cell 10 or the second prior art cell and that can be driven with the waveforms of FIG. 4.